I am a newbie on linux and dont understand many things unless given a line by line instructions. I am trying but linux does take some getting used to.

I have installed ubuntu from live usb stick. Now I cannot boot my system without the usb being plugged in. There are answers whereby certain commands need to be run. For any command that I run with sudo (prefix) the terminal asks me for a password which I dont know.

What do I do? How do I fix it such that the booting happens without usb being plugged in.

So far I have just been booting from USB. Please help a recent converted from Windows.

The password that's being asked for is your authentication password: it's the one you set during the installation process when the installer asks for your details. Also the one you use to log into your Ubuntu desktop. :)

Now, I'm a 2 month old Ubuntu convert myself and I don't come from a technical background, but it looks as though your Ubuntu bootloader (the program which is responsible for starting up your operating system) somehow got installed onto your flash drive instead of your HDD, as it should have been.

The topic that Anwar linked you to above sounds great to me. If that doesn't work, I guess you could try re-installing Ubuntu- and if you do, please go through the method described in the tutorial I'm linking you to; I used it myself, as did a friend and it worked wonders.
Ubuntu 12.04 installation tutorial